Dr Hankin, The Problem of Soaring Flight 223 



on a curved course. When watched through a binocular they 

 resemble a number of dead birds pinned on a blue wall (IV, pp. 

 60 and 61). 



The regularity of soaring flight thus shown conspicuously by 

 cranes, and less strikingly by all soaring animals that fly in groups, 

 furnishes a clear proof that the energy involved does not come from 

 any chance or irregular currents of air. 



Two alternatives appear to be suggested. Either such regular 

 flight is due to undiscovered wing movements or its cause must be 

 some condition widely and uniformly distributed in the atmosphere. 



3. The flight of the puttung. 



The puttung is a kite made of paper and bamboo in common use 

 by Indian boys. 



The chief peculiarity of its flight is that, in a suitable wind, it 

 flies vertically over its string and when so flying its flight is par- 

 ticularly stable. To achieve this result the front limb of the bridle 

 that attaches it to the string must be a little shorter than the hind 

 limb. 



This mode of flight can only be explained by the supposition 

 that this kite takes energy from the air after the manner of the 

 soaring bird. 



Its flight resembles that of the soaring bird in that it is more 

 liable to show lateral instability late in the afternoon than at other 

 times of the day. 



When struck by a gust it may, for a few seconds, fly up wind in 

 advance of the vertical. Similarly, birds and dragon-flies soaring 

 up wind when struck by a gust often show an increase of speed. 



The structure of this kite is such that when it is exposed to wind 

 pressure there must be a ridge on its under surface that lies trans- 

 verse to the line of flight. In all the more efficient soaring animals 

 ridges transverse to the line of flight are present on the under 

 surfaces of the wings (IV, pp. 242 and 341). 



If it is admitted that the flight of this kite is an instance of 

 soaring then obviously the idea of undiscovered wing movements 

 must be given up and, also, a means is indicated by which the 

 phenomena of soaring may be submitted to an experimental 

 investigation. 



4. Soaring flight not due to undiscovered wing movements. 



(a) Flying-fishes can check their speed by hanging their hind 

 wings downwards. They habitually do so towards the end of a 

 flight if it has been carried out at high speed. But in the highly 

 soarable monsoon winds they often use this air brake during the 

 whole of a flight and, as may be seen, with this brake in action 



